Mature plasma cells comprise the majority of tumor cells in multiple myeloma (MM), but we recently reported that these cells have little to no clonogenic growth potential either in vitro or in vivo. Instead, we found that MM growth was driven by clonally related memory B cells that are capable of recapitulating disease in immunodeficient mice and are relatively resistant to standard anti-MM therapies. These functional properties suggest that these clonotypic B cells are MM cancer stem cells (CSC) and play a central role in disease relapse and progression. However, it is unclear whether our laboratory findings apply to the pathogenesis of the disease since little data exists in MM or any human malignancy that CSCs are actually clinically relevant. Proof that the changes in the frequency and/or persistence of CSCs are associated with clinical outcomes or that the targeting and inhibition of CSCs improves survival rates would provide the strongest evidence for their clinical relevance and promote their further study. To this end, we have developed laboratory assays that are capable of serially quantifying MM CSCs over the course of treatment and our early data suggest that changes in MM CSC frequency are associated with progression free survival. We have also identified MM CSC targeting strategies based on their expression of CD20 and enhanced telomerase activity and have initiated pilot clinical trials utilizing the anti-CD20 antibody rituximab and the novel telomerase inhibitor imetelstat. Although clinical activity was limited in these early phase trials, results from our correlative studies have provided important insights regarding anti-CD20 and telomerase inhibitor based approaches in MM and allowed us to modify these approaches. The overall goal of this project is to determine whether or not MM CSCs are clinically relevant, and we believe that our previous studies now place us in an ideal position to address this question by both correlating their frequency with clinical outcomes and determining the clinical impact of novel CSC-targeting strategies. These findings may have important implications for both MM and the CSC field in general, and we propose to: 1). Examine the relationship between MM CSC frequency and clinical outcomes in patients treated with state of the therapies; 2). Determine the effects of B cell directed radioimmunotherapy in MM; and 3). Determine the activity of telomerase inhibition in MM.